The whole point to limiting signeage is that the traffic regulators must keep the rules simple enough so that they're short enough so that someone driving past the sign can fully understand it without stopping and/or endangering traffic.
Need temporary exceptions for snow removal? Change the signs. Can't afford to change the signs so often? Don't make so many and complicated exceptions or schedules.
You can make a really reasonable system by only using simple signs in the form "[park here/don't park here/pay-park here] IF [time-range-sign] OTHERWISE default". Many countries are successfully doing this - my home city is just fine and most of the parking signs are either (a) no parking here; (b) pay-parking on workdays 7:00-21:00 (otherwise default free-parking); (c) no-parking on 7:00-19:00 on workdays (to free up a lane for traffic) or (d) parking forbidden except permit #xyz.
Need temporary exceptions for snow removal? Change the signs. Can't afford to change the signs so often? Don't make so many and complicated exceptions or schedules.
You can make a really reasonable system by only using simple signs in the form "[park here/don't park here/pay-park here] IF [time-range-sign] OTHERWISE default". Many countries are successfully doing this - my home city is just fine and most of the parking signs are either (a) no parking here; (b) pay-parking on workdays 7:00-21:00 (otherwise default free-parking); (c) no-parking on 7:00-19:00 on workdays (to free up a lane for traffic) or (d) parking forbidden except permit #xyz.